Stress and the Adrenocortical Control of Epinephrine Synthesis Richard J

Stress and the Adrenocortical Control of Epinephrine Synthesis Richard J. Wurtman Psychologic states produced by environmental or physiologic stresses are usually associated with hypersecretion of adrenal hormones. particularly epinephrine and the glucocorticoids (hydrocortisone in humans or corticosterone in rats). A common mechanism links the secretion of these hormones. even though the adrenal medulla and cortex have different embryologic origins and biochemical properties and very different mechanisms controlling their secretory activities. ie. a cholinergic nervous input stimulates medullary secretion while a hormone. corticotropin (ACTHI. activates secretion from the cortex. This mechanism is made possible by an intra-adrenal portal vascular system. which provides the medulla with uniquely high concentrations of glucocorticoids. These high concentrations are needed to induce the medullary enzyme. phenylethano- lamine-N-methyltransferase (PNMT). which controls the synthesis of epinephrine from norepinephrine. By suppressing glucocorticoid secretion. pituitary failure compromises epinephrine synthesis and decreases the rate at which epinephrine is secreted; in contrast. prolonged chronic stress can enhance epinephrine synthesis and secretion within the adrenal. the brain. or both organs. This control mechanism could be involved in the long-term consequences of stress. Copyright 2002. Elsevier Science (USA). All rights reserved. HE CLASSIC PERIPHERAL manifestations of severe rod, to determine whether pituitary failure could be shown to T environmental or physiological stress-increased plasma affect the synthesis or secretion of this catecholamine. These levels of epinephrine and hydrocortisone (in humans) or corti- studies led to the discovery that the very high concentrations of costerone (in rats)-result from hypersecretion of these hor- glucocorticoids that the adrenal cortex normally delivers to the mones from the adrenal medulla and cortex, respectively. A adrenal medulla (via the adrenal's portal vascular system) in- common mechanism links these processes, even though the duce the enzyme, PNMT, that is rate-limiting in epinephrine medulla and cortex have different embryologic origins and synthesis. By suppressing glucocorticoid secretion, pituitary biochemical properties, and very different chemical signals (the failure not only compromised epinephrine synthesis, but also neurotransmitter acetylcholine and a pituitary hormone, corti- decreased the levels of epinephrine in the adrenal medulla and cotropin [ACTH), respectively) control their secretory activi- the rate at which epinephrine is secreted in response to hypo- ties. For several decades, on the basis of experiments per- glycemia.5 formed on rats, it had been thought that circulating epinephrine A severe prolonged stress, which chronically elevates glu- acted directly on the pituitary to control the release of ACTH. cocorticoid secretion, can have opposite effects. In this formulation the first step in the adrenal "stress response" ADRENAL MORPHOLOGY AND EPINEPHRINE SYNTHESIS was thought to be the liberation of epinephrine; this in turn released ACTH, leading to hypersecretion by the <ldrenalcor- On the basis of the adrenal's unique property of containing 2 tex. However, subsequent experiments on humans failed to distinct organs, of which one (the cortex) surrounds the other demonstrate any consistent effect of circulating epinephrine on (the medulla), it had been suspected for a while that one .of ACTH secretion. Hence, it is no longer believed that epineph- these organs played some special role in the physiology of the rine controls glucocorticoid secretion. Rather, the converse is other. In 1953, Coupland, an English anatomist, drew attention true: glucocorticoids control epinephrine synthesis, and thereby to the correlation between the morphology of the mammalian . affect its secretion.1.2 adrenal gland in any particular species and its contents of This perception had its origins in a patient that I saw as a epinephrine or norepinephrine.6 He noted that in species in student in 1965. She had become unusually sensitive to endog- which the adrenal cortex and medulla are juxtaposed (ie, in rats enous insulin after suffering a pituitary infarction during a and humans), epinephrine is the major medul!ary catechol- difficult labor. Several hours after eating a carbohydrate-rich amine, while in those which the medullary chromaffin tissue meal she would develop persistent hypoglycemia, sometimes lacks a cortical envelope, it contains little or no epinephrine. In even leading to seizures. the rabbit, part of the medulla is surrounded by cortex, and this part contains essentially all of the medullary epinephrine. On The enhanced insulin sensitivity seen in patients with pituitary insufficiency had generally been attributed to the absence the basis of these findings, Coupland suggested that the adrenal cortex secreted a "methylation factor" that influenced the N- of a hyperglycemic factor normally secreted from the pituitary methylation of norepinephrine to form epinephrine. (eg, growth hormone) or to the impairment in gluconeogenesis that might be expected to result from the ACTH deficiency and resulting decrease in glucocorticoid secretion.3.4However, this explanation seemed suspect, given that growth hormone or From the Massachusetts Institute of Technology, Cambridge, MA. ACTH need many hours to raise plasma glucose levels. More Supported by grants from the National Institutes of Health. likely, hypopituitary patients also lacked the substance that Address reprint requests to Richard J. Wurtman, MD, Cecil H. Green Distinguished Professor, Massachusetts Institute of Technology, normally acts most rapidly to restore plasma glucose levels 77 Massachusetts Ave, E25-604, Cambridge, MA 02139. after induction of hypoglycemia. Since epinephrine seemed the Copyright 2002, Elsevier Science (USA). All rights reserved. best candidate for this hypothetical fast-acting hyperglycemic 0026-0495/02/5106-1004$35.00/0 agent, we designed studies, in collaboration with Julius Axel- doi: I 0.1 053/meta.2002.33185 Metabolism. Vol 51, No 6, Suppl 1 (June), 2002: pp 11-14 11 -- 12 RICHARD J. WURTMAN Table 1. Effect of Hypophysectomy, ACTH, and Dexamethasone dulla in much lower concentrations than those available after on PNMT ACTH administration). T reatment* Adrenal Weight (mg/pairl PNMT (,..mol/pair) GLUCOCORTICOIDSAND EPINEPHRINESYNTHESIS Control 68.9 :!: 3.4t 6.16 :!: 0.66t Hypophysectomy 26.5 :!: 2.1 1.46:!: 0.11 There were at least 3 possible mechanisms by which ACTH Hypophysectomy plus treatment could have restored PNMT activity in the hypophy- ACTH 46.7 :!: 1.5t 4.76 :!: 0.26t sectomized rat: Hypophysectomy plus I. The adrenal cortex might have contained an unrecog- dexamethasone 26.0 :!: 2.3 7.80 :!: O.44t nized epinephrine-forming enzyme, in which case the *Animals were sham-operated or hypophysectomized and killed 17 decline in cortical mass after hypophysectomy, and its to 21 days later. Some of the hypophysectomized rats were given restoration with ACTH treatment, would be expected to ACTH or dexamethasone for 6 days prior to autopsy. cause parallel changes in PNMT activity. tp < .001 v hypophysectomy. 2. ACTH could have acted directly on the adrenal medulla to enhance PNMT activity. 3. ACTH could have stimulated the secretion of glucocor- In 1959, Kirshner identified the adrenomedullary enzyme, ticoids by the adrenal cortex; these compounds, deliv- PNMT, responsible for the synthesis of epinephrine from nor- ered to the medulla by the intra-adrenal portal vascular epinephrine.7 Subsequently, Axelrod demonstrated that most of system, could then have increased PNMT activity. the epinephrine-forming ability found in adult mammals was To examine the first possibility, we separated the cortex and localized within the adrenal glandS; hence, most of the epineph- medulla from adrenals of hypophysectomized and normal an- rine released into the circulation had very likely been produced imals, and assayed them individually for PNMT. We found that by PNMT within the adrenal medulla. An isotopic assay was the cortex normally contains less than 10% of the epinephrine- subsequently developed for PNMT, which simplified measur- forming activity of the medulla. (Since no capsule separates the ing the effects of various physiologic states and drugs on rat cortex from the medulla, it is possible that even this 10% epinephrine biosynthesis. represented contamination of-the cortic~1sample with medul- PITUITARYACTHAND EPINEPRINESYNTHESIS lary tissue.) To rule out the possibility that ACTH acted directly on the Based on the pathologically prolonged hypoglycemia of our adrenal medulla, it was necessary to show that when the level hypopituitary patient, we initiated studies to see whether the of glucocorticoids available to the medulla was held constant, glucocorticoid hormones were the unknown "methylation fac- changes in the rate of ACTH secretion had no effect on PNMT tor." Hypophysectomy was found to be associated with a activity}OThis was done in 2 ways. First, normal rats were marked reduction in epinephrine-forming ability (Table I)} treated with large doses of dexamethasone, a highly potent This reduction was of even greater magnitude than the fall in synthetic glucocorticoid. This dose depressed pituitary secre- adrenal weight (which was due largely to a reduction in the tion of ACTH (as demonstrated by a fall in adrenal weight, mass of the adrenal cortex). When hypophysectomized

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